Scientific Method —

Squeezing precision out of magnetic detectors

Physicists may have found a route to making the world's most sensitive …

Magnetic fields play some really important roles in modern life. We store information in the alignment of magnetic fields on hard disks, we use the molecular response to magnetic fields to determine the shapes of proteins and image inside the body. In these cases, we apply a magnetic field and measure a response to that applied field.

However, a lot of information could potentially be gained just by measuring naturally occurring magnetic fields. But, in biomedical applications, anyway, these fields tend to be pretty weak. Once you consider that the information you want is buried in the fluctuations of that field, you realize that a pretty sensitive and fast magnetometer is the order of the day if you actually intend to detect anything.

Usually, modern magnetometers are based on the Hall effect, where a magnetic field bends a current flowing through a conductor, creating a small voltage perpendicular to the current flow. The problem is that the voltage is subject to all the usual environmental noise from surrounding electronic devices so, for better sensitivity, you need to use an optical method.

Reading magnetic fields with light

This is exactly what a research team (Wolfgramm and co-workers) at Institut de Ciencies Fotoniques in Castelldefels, Spain have recently published in Physical Review Letters. The idea is very simple: when light travels through particular substances with an external magnetic field applied, the polarization of the light is changed. This is usually a pretty small effect unless you choose the right materials. In this case, the right materials are two isotopes of rubidium, in vapor form.

Because it has an unpaired electron, rubidium will align itself to a magnetic field and, in doing so, it will change the polarization of a light beam passing through it. Measure the polarization, and you know the magnetic field strength. Do it twice at different angles, and you know the magnetic field strength and orientation.

However, the experiment is a bit more complicated than that. First, the degree of polarization rotation depends on the wavelength of the light—for most wavelengths, the effect is incredibly weak. To overcome this problem, the researchers chose a wavelength of light that is almost, but not quite, tuned to an electronic resonance of the rubidium vapor. As a result, the polarization rotation is very strong.

So far, so good. But we want high sensitivity and high accuracy, and that's not easy. If the laser wavelength changes just a tiny amount, then that will change the amount of polarization rotation and add noise to the measurement. Lasers also have a certain amount of polarization noise. In fact, polarization states are subject to Heisenberg's uncertainty principle, meaning that there is a joint minimum to the accuracy with which the total polarization state can be measured.

Clearing Heisenberg's hurdle

The first problem is dealt with by using not one isotope of rubidium, but two. The heavier, 87Rb, is used to rotate the polarization. The second, lighter isotope (85Rb), is present at less than one percent concentration. The laser is tuned so that its wavelength sits exactly on the electronic transition of the lighter isotope. Now, by monitoring the amount of absorption of the light beam, the laser wavelength can be constantly corrected so that it stays very close to where it is wanted.

The lighter isotope absorbs light at a slightly different wavelength to that of the heavier one, so tuning the wavelength to the light one allows researchers to keep the wavelength of the laser close to, but not right on top of, the electronic resonance of the 87Rb.

On to the second problem. The polarization has an inherent amount of noise, and this seems an insurmountable problem. But, in fact, there is a loophole and this is where the researchers focused. The noise issue only applies to the complete polarization state. If you only need part of the polarization state, then you can bitch slap the uncertainty principle.

To do this, you create a source of light that has an extremely well defined polarization along a particular orientation (say parallel to the lab table). The price you pay for this is that the polarization along all other orientation becomes very poorly defined. This light is called squeezed, and in practice you get a periodic oscillation in the noise for each polarization.

So at any particular time, one polarization might be well defined, but the other is not, and the two alternate. Once you know the period of oscillation, you can slice the data from your measurement to only use the least noisy polarization states.

The end result is a rather complicated set of equipment that fills up an optical table. But, it also demonstrated that using squeezed light states improves the magnetometer's sensitivity by a factor of just over two, to something around the 3nT mark (with a time resolution of ~1s).

Unfortunately, this is still 7 orders of magnitude less sensitive than the best examples of this work. The reason for the difference is due to how the rubidium was used. In experiments that reach 1fT resolution, the rubidium are first prepared so that their magnetic moment are all pointing in the same direction; Wolfgramm and co-workers used unpolarized rubidium.

There is certainly no reason why the two cannot be combined, which, I imagine, is what they are planning on doing. Nevertheless, I simply cannot envision such a setup being used in medical diagnosis or suchlike. Certainly, it could end up being used in scientific applications, and, in particular, highly sensitive space-borne magnetometers might be where these sorts of instruments first demonstrate their true potential. But, until it becomes a lot more compact, it isn't going anywhere.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku